The commercial baking industry has long sought additives which improve softness and prevent staling of baked products. The ordinary consumer equates softness with freshness and any change in a baked product during storage which develops firmer texture is viewed as undesirable. Such change is, however, inevitable as time dependent effects which are largely associated with the starch component of flour begin to occur immediately after baking. This staling phenomenon ultimately restricts the lifetime of a baked product prior to use and has major impact on the economics associated with sales and distribution.
While much study has been devoted to staling of baked products, and much progress has been made in the understanding of the process, there are still many unanswered questions regarding the staling mechanism. Flour contains predominantly two complex macromolecular structures--gluten and starch fraction. Each contributes both functional and nutritional aspects to baked products. Gluten is believed to be more dominant with respect to formation and stabilization of the developing gas cell structure, while starch is believed to be important in water absorption and stabilization of the skeletal cellular network that remains after the baking cycle. Mechanically developed gluten (produced during dough mixing) consists of a microscopically stranded network of thread like structures surrounding a discontinuous phase of dispersed starch granules. During the fermentation stage microscopic gas pockets nucleate and are stabilized by the continuous phase comprised of the gluten starch network. Ultimately a dispersed foam structure is formed. On baking the gluten is denatured and the starch is gelatinized about a thermally expanding gas cell to form a stable film compartmentalizing the dispersed gas cells. After baking the film begins to harden as the two principal starch components, amylose and amylopectin, begin to pass through their respective glass transition temperature ranges. The final baked product is best described as a microscopically networked sponge. Over time, profound physical changes begin to occur largely due to moisture migration from the central interior toward the periphery of the baked dough piece and oligomeric restructuring of the amylopectin and amylose starch components. Some ordering of the now partially denatured gluten structure may also begin to occur.
There are three recognized ways to enhance softness and abate staling. Each has some limitation. Firstly, it has been found that alpha monoglycerides of C-16 and C-18 fatty acids (particularly glycerol monosterate, GMS) are very effective in the prevention of retrogradation (crystal formation) of the amylose fraction. Amylose forms strong helical clathrate structures about GMS which prevents or interrupts substantial crystalline domains from forming with this high molecular weight alpha 1-4 linear glucan. This is largely a one time fix as the effect is immediate after baking when there is still some mobility for the amylose chains to configurationally encase the GMS. There is a limit to the effectiveness of GMS, as the available sites for clathrate formation are rapidly saturated as retrogradation to form GMS inaccessible crystalline domains is kinetically competitive.
The second recognized means of abating staling is the use of amylolytic enzymes. There are at least three important mechanisms of enzyme action which can occur during or after the baking cycle, depending on the heat stability of the enzyme type in question. Alpha amylases are complex enzyme mixtures usually containing several alpha 1-4 glucan hydrolase activities. They basically catalyze the random hydrolysis of long chain starch segments, breaking the chain endolytically to effect depolymerization. They are particularly effective on the linear amylose chains and extended linear segments of the branched amylopectin fraction. In both cases the ability to retrograde and form intermolecular crystalline regions is abated by depolymerization to shorter chain length and generation of a less interactive species designated dextrins. Alpha amylases are broadly classified by their microbial source and characterized by their thermal stability. Fungal alpha amylases are typically thermally labile and are inactivated by the temperatures achieved during the baking cycle. Bacterial alpha amylases are more thermally stable and can largely survive the baking cycle. This can be a problem if their concentration is too high as the residual activity after baking can lead to "gummy" baked products due to continued and excessive starch hydrolysis. In general there is a limit to the use of amylolytic enzymes as too much activity leads to sticky doughs or later a gummy baked product. A second class of amylolytic enzyme is designated glucoamylase. This enzyme operates on starch and dextrins in an exolytic fashion to hydrolytically cleave the pendent glucose moiety sequentially along the alpha 1-4 glucan chain starting at the reducing sugar end. This activity may be present in commercial alpha amylase products. It is not a major contributor to amylase function but can control dextrin accumulation which contributes to sticky doughs. It is thermally labile and inactivated by the baking process. A third class of amylotic enzymes is designated beta amylase or maltogenic amylase. These enzymes, like glucoamylase, are exoactive cleaving the dissacharide maltose sequentially from the reducing end of the polysaccharide chain. Amylose is ultimately converted to maltose, a readily fermented carbohydrate by yeast. Amylopectin is converted to a species denoted as a beta limit dextrin. Beta amylases cannot proceed beyond the 1-6 branch point found in amylopectin, hence the potentially intermolecular interactive alpha amylose chains which extend from the highly branched amylopectin structure are selectively degraded to maltose. Beta amylases are usually of fungal or vegetable origin and are inactivated during the baking cycle.
The third means of softening bread and hence delaying staling is simply to add more water to the dough. While much of the water absorption is intimately associated with the starch granules present in the starch component of wheat flour, excess water which serves as a mediator of moisture depletion in the baked dough piece is largely controlled by the gluten fraction. Again there is a limit to water incorporation as the dough ultimately becomes sticky and cannot be processed by automated equipment. Typically white bread doughs range from 55 to 65.about. absorption on flour; however, variety breads and specialty diet bread doughs can have less or more absorption, respectively.
The use of cellulose in bread and other baked products is not entirely new. Powdered and ground cellulosic substances have been used for the production of low or reduced calorie products for over thirty years. These forms of cellulose, however, constitute nutritionally unavailable carbohydrate that functions as an inert bulking agent replacing flour. The levels of cellulose incorporated, based on flour, are relatively high, ranging from 15 to 25%. At these high levels of flour dilution, additional gluten is required for dough performance. Moisture content in diet breads is somewhat elevated due to higher water absorption by the cellulose particles versus flour. However the additional moisture is intimately associated with hydration of the cellulose particle domains and not available to the surrounding continuous starch/gluten matrix. The incorporation of large amounts of ordinary refined cellulose into a reduced calorie bread does not impart any significant anti-staling or extraordinary softening effect. Insofar as is known, it has not previously been proposed to add cellulose to baked products as a means to improve softness or prevent staling.